In an internal combustion engine, engine valve actuation is required in order to produce positive power, and may also be used to produce engine braking and/or exhaust gas recirculation (EGR). During positive power, one or more intake valves may be opened to admit air into a cylinder for combustion during the intake stroke of the piston. One or more exhaust valves may be opened to allow combustion gases to escape from the cylinder during the exhaust stroke of the piston.
One or more exhaust valves may also be selectively opened to convert, at least temporarily, the engine into an air compressor for engine braking operation. This air compressor effect may be accomplished by either opening one or more exhaust valves near piston top dead center (TDC) position for compression-release type braking, or by maintaining one or more exhaust valves in a relatively constant cracked open position during much or all of the piston motion, for bleeder type braking. In either of these methods, the engine may develop a retarding force that may be used to help slow a vehicle down. This braking force may provide the operator with increased control over the vehicle, and may also substantially reduce the wear on the service brakes. Compression-release type engine braking has been long known and is disclosed in Cummins, U.S. Pat. No. 3,220,392 (November 1965), which is hereby incorporated by reference.
The braking power of a compression-release type engine brake may be increased by selectively actuating the exhaust valves to carry out brake gas recirculation in combination with compression release braking. Brake gas recirculation (BGR) can be accomplished by opening an exhaust or auxiliary valve near bottom dead center of the intake or expansion stroke of the piston and keeping the exhaust or auxiliary valve open during the first portion of the exhaust or compression stroke of the engine. Opening the exhaust or auxiliary valve during this portion of the engine cycle may allow exhaust gas to flow into the engine cylinder from the relatively higher-pressure exhaust manifold. The introduction of exhaust gases from the exhaust manifold into the cylinder may pressurize the cylinder with a charge faster than it would otherwise occur during the compression stroke. The increased gas pressure in the engine cylinder may increase the braking power produced by the compression-release event.
There are many different systems that may be used to selectively actuate an exhaust or auxiliary valve to produce BGR and compression-release events. One known type of actuation system is a lost motion system, described in the aforenoted Cummins patent. An example of a lost motion system and method used to obtain engine braking and brake gas recirculation is disclosed in Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) which discloses a method of conducting brake gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the intake stroke, and which is hereby incorporated by reference. Gobert uses a lost motion system to enable and disable compression-release braking and brake gas recirculation. The system disclosed in Gobert opens the exhaust valve near bottom dead center of the intake stroke for a BGR event, closes the exhaust valve before the midway point of the compression stroke to terminate the BGR event, and opens the exhaust valve again near top dead center of the same compression stroke for a compression-release event. As a result, the exhaust valve actuated in accordance with the Gobert system must be rapidly seated and unseated between the BGR and compression-release events.
In many internal combustion engines, the intake and exhaust valves may be actuated by fixed profile cams, and more specifically, by one or more fixed lobes or bumps that are an integral part of each cam. The cams may include a lobe for each valve event that the cam is responsible for providing. The size and shape of the lobes on the cam may dictate the valve lift and duration which result from the lobe. For example, an exhaust cam profile for a system constructed in accordance with the aforenoted Gobert patent may include a lobe for a BGR event, a lobe for a compression-release event, and a lobe for a main exhaust event.
It may also be desirable to increase the exhaust back pressure in the exhaust manifold during engine braking. Higher exhaust back pressure may increase gas mass and pressure in the engine cylinder available for engine braking, and thereby increase braking power. Increased exhaust back pressure, however, may undesirably increase the force required to open the exhaust valve for a compression-release event because the opening force applied to the exhaust valve must exceed the increased pressure in the engine cylinder resulting from the increased exhaust back pressure. To some extent the increased exhaust back pressure may also increase the pressure applied to the back of the exhaust valve, which may counter-balance the increased pressure in the cylinder and thus reduce the loading on the exhaust valve opening mechanism used for the compression-release event.
Increasing the pressure of gases in the exhaust manifold may be accomplished by restricting the flow of gases through the exhaust manifold. Exhaust manifold restriction may be accomplished through the use of any structure that may, upon actuation, restrict all or partially all of the flow of exhaust gases through the exhaust manifold. The exhaust restrictor may be in the form of an exhaust engine brake, a turbocharger, a variable geometry turbocharger, a variable geometry turbocharger with a variable nozzle turbine, and/or any other device which may limit the flow of exhaust gases.
Exhaust brakes generally provide restriction by closing off all or part of the exhaust manifold, thereby preventing the exhaust gases from escaping. This restriction of the exhaust gases may provide a braking effect on the engine by providing a back pressure when each cylinder is on the exhaust stroke. For example, Meneely, U.S. Pat. No. 4,848,289 (Jul. 18, 1989); Schaefer, U.S. Pat. No. 6,109,027 (Aug. 29, 2000); Israel, U.S. Pat. No. 6,170,474 (Jan. 9, 2001); Kinerson et al., U.S. Pat. No. 6,179,096 (Jan. 30, 2001); and Anderson et al., U.S. Pat. Appl. Pub. No. US 2003/0019470 (Jan. 30, 2003) disclose exhaust brakes for use in retarding engines.
Turbochargers may similarly restrict exhaust gas flow from the exhaust manifold. Turbochargers often use the flow of high pressure exhaust gases from the exhaust manifold to power a turbine. A variable geometry turbocharger (VGT) may alter the amount of the high pressure exhaust gases that it captures in order to drive a turbine. For example, Arnold et al., U.S. Pat. No. 6,269,642 (Aug. 7, 2001) discloses a variable geometry turbocharger where the amount of exhaust gas restricted is varied by modifying the angle and the length of the vanes in a turbine. An example of the use of a variable geometry turbocharger in connection with engine braking is disclosed in Faletti et al., U.S. Pat. No. 5,813,231 (Sep. 29, 1998), Faletti et al., U.S. Pat. No. 6,148,793 (Nov. 21, 2000), and Ruggiero et al., U.S. Pat. No. 6,866,017 (Mar. 15, 2005), which are hereby incorporated by reference.
Compression-release engine braking is not the only type of engine braking known. The operation of a bleeder type engine brake has also long been known. During bleeder type engine braking, in addition to the normal exhaust valve lift, the exhaust valve(s) may be held slightly open continuously throughout the remaining engine cycle (full-cycle bleeder brake) or during a portion of the cycle (partial-cycle bleeder brake). The primary difference between a partial-cycle bleeder brake and a full-cycle bleeder brake is that the exhaust valve is closed for the former during most of the intake stroke.
Usually, the initial opening of the braking valve(s) in a bleeder braking operation is far in advance of the compression TDC (i.e., early valve actuation) and then lift is held constant for a period of time. As such, a bleeder type engine brake may require much lower force to actuate the valve(s) due to early valve actuation, and generates less noise due to continuous bleeding instead of the rapid blow-down of a compression-release type brake. Moreover, bleeder brakes often require fewer components and can be manufactured at lower cost. Thus, an engine bleeder brake can have significant advantages.